Pulse generator



Patented Apr. 22, 1947 calato;

orifice.

PULSE GEIEBATOB Elmo E. Crump, East Orange, and Lawrence'G.

Ker-sta, Madison, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York. N. Y., a corporation ot New York Application November 3U, 1943, Serial No. 512,294

This invention relates to pulse generators, particularly thosefor keying or modulating ultrahigh frequency radio transmitters for the transmission of recurrent, short pulses of high power. Various systems, for example, distance measuring and object locating ysystems employ high l power recurrent pulses of ultrahigh frequency radio waves. Such pulses may be of very short duration compared to the interval between` eration of this type of system is that of double" energy storage, the energy from the prime source;y being first stored in the magnet field of an iiiductive system and then transferred to a storageL capacitor from which it is discharged to the load circuit. Accordingly, the active portion oi' 'the 'cycle oi' operation comprises three periods, the inductance charging period, the capacitor charg. ing period and the pulse period. In order to provide this cycle of operation there are required two discrete switching operations or their equlv alents; ilrst, for the transfer of the energy from the magnetic to the electric storage, and second,

lfor the ultimate discharge to the load during pulse period.

In the Shockley application one of the types of circuits for accomplishing the {second switching or transfer operation employs a non-linear in.; ductor. In the operation of such a system the non-linear inductor functions as a switch elet ment controlled by the current ilowingtherethrough, by reason of the fact that its inductancev undergoes a sharp change from a high to a negligibly low value. as its core passes from the nonsaturated to the saturated condition. The non-e linear inductor has current supplied to it during the capacitor charging period and must be main tained in the unsaturated condition during this period. moient operationrequires that it reach. the saturation point at the time the storage capacitor assumes its maximum charge. This and other factors such as the limitation of the cur rent drain through the non-linear inductoi have 15 Claims. (Cl. 171-9'0 required not only an accurate proportioning of circuit elements but also the use of auxiliary cirrcuit features. In some systems of this general type the use of a supply of biasing current to the non-linear inductor has been found to give desirable results. The application of such systems may have some limitations particularly where power supply requirements are a controlling tactor.

In accordance with a feature of the present invention, the operation oi the non-linear inductor of a pulse generating system of the type'oi the above Shockley application is coutrolled'by the use oi' a capacitor-resistor combination. The can pacitor provides a low impedance path for supplying the output pulse to the load. At the same time its capacity is made low enough so that it does not carry the nondinear inductor far into the saturated region during the inductive charging period which would otherwise waste energy and impose operating limitations on the vacuum tube used to provide that current path. The resistor functions during the inductance charging period tolicld the current in the nondinear inductor to such a value that the saturation time of the coil will correspond to the required to for the storage capacitor to assume its maximum charge, and in addition 'to provide sufiicient resistance to limit that current to a value below saturation. These requirements for both the capacitor and resistor can be met with relatively uncritieal values.

These and other objects. features and aspects ci? the invention will be better understood hy reference to the following detailed description in connection with the drawing, in which:

Fig. 1 is a schematic circuit diagram of one em- 'bodiment o! the invention, and I Figs. 2 and 3 are graphs illustrating the operation of the invention:

In the system-of Fig. l the invention is employed for producing recurrent short pulses of high power ultra-high frequency radio4 waves. For the purposes of ilustration, the ultra-high frequency generator is shown as a multicavity magnetron I0 oi' the type disclosed in United States Patent 2,063,342 to. Samuel, December 8, 1938. In this type of oscillator the anode I l forms an external sheath or enclosure which it is desirable to maintain at ground potential. When the tube is subjected to a. magnetic field suppliedby the magnet iii and a high direct current voltage is impresses between the anode il and the electron emitting cathode il, there are produced power oscillations of a frequency determined by the internal structure of the device as described in more detail in the Samuel patent. These os cillations may be picked up by a loop Il and transmitted through a coaxial line I5 to an antenna (not shown).

The purpose of the remainder of the circuit is to produce the recurrent direct current pulses that are impressed between the anode II and cathode I2 for causing the corresponding pulses of ultra-high frequency oscillations. It is to this pulse generating circuit that this invention is particularly directed. i

The prime power source is the battery I', though obviously some other source such as a generator or rectifier will be found preferable in most practical applications of the invention. The iiow of current from the battery Il is controlled by the vacuum tube I9 whose essential function in the system is that of a switching element. 'I'his tube is normally biased to plate current cut-oi! by the negative voltage applied to its grid by the -i battery 24. The operation of the tube is controlled by the positive voltage supplied to the grid of tube 9 by the square wave generator 22. (Alternatively the tube I9 might be maintained in the cut-off condition by a negative voltage from the square wave generator 23 and rendered conductive during the portion of the cycle in which the voltage is zero or positive.)

A linear inductor I8 is connected in series with the battery Il in the plate circuit of the tube I9. During the inductive charging period energy is stored in the magnetic field of this inductor. After the tube I2 is cut off this energy is transferred to the storage capacitor 20. A diode 2I provides a path for this charging current and prevents pulses of incorrect polarity from appearing across the magnetron.

A non-linear inductor 22 is provided for accomplishing the second switching or energy transfer operation namely, the discharge of the storage capacitor 20 through the magnetron I0. This inductor 22 has the special characteristic that its inductance is high for low flux densities and changes quite sharply to negligibly low value in its saturation condition. One type of construction for such an inductor may comprise a toroidal winding on a core of spirally wound tape of the molybdenum permalloy type. A capacitor 25 is connected in series with the inductor 22 and a resistor 28 is connected between resistor function to aid in the control of the nonlinear inductor 22 in performing the switching function in a manner to give the desired operation. This action will be explained in the detailed description of a cycle of operation of the system which will be given below.

The biiilar inductor 2l' provides a means for suppiying heating current to the cathode I3 of the magnetron without the use of a more expensive insulating transformer. This inductor has a high impedance for high frequencies representing the pulse length and a low impedance at the low power frequency of the alternating current source furnishing the heating current. The inductor also functions in combination with the charging diode 2i to prevent oscillations in the pulse circuit as is explained in the Shoclzley application referred to above.

The operation of the system will now he described with reference to the explanatory diagrams of Figs. 2 and 3 in which various voltages andY currents in the system are plotted against time. In this description it is assumed that the circuit has been operating for several cycles prior portion of the cycle prior to t1 the tube I9 has been blocked and the capacitor 25 has become charged substantially to the voltage of the battery I1. At the time t1 the voltage from the source 23 becomes positive and remains positive until the time tz, as shown in Fig. 2. During this period. from t1 to tz, the tube I9 is conductive and current iiows through its space path. This current (I1 of Fig. 3) builds up slowly owing to the high inductance provided by the inductors IB and 22 in the plate circuit of tube I9. The main component of this current is supplied by the battery I'I through the inductor I8. The component through the inductor 22 is first supplied mainly by the charge on the capacitor 25. 'Ihe capacity of this element is so chosen that its charge is quickly dissipated without carrying the inductor 22 far into the saturation region. The effect of this discharge of the capacitor 25 is shown by the hook (A) in the current curve of Fig. 3. From this point on the current through the inductor 22 is determined by the resistor 28 which is given such a value as to hold the current through inductor 22 at about the saturation point or just below.

At the time t: the grid of the tube I9 is driven to such a negative value that the plate path is cut oil sharply. The inductive current must find another path namely through the storage capacitor 20 and the charging diode 2l. As a result the capacitor 20 is charged by the energy stored in the inductor I8 during the previous period. During this period the current in the non-linear inductor 22 is also changing. The voltage building up on the capacitor 20 first decreases the current in inductor 22, then reverses it, finally driving the inductor 22 into its saturation region in the sense opposite to that which it approached during the time the tube I9 was conducting. As soon as this saturation condition is reached the impedance of the non-linear inductor 22 drops to a very low value providing a low impedance path for the discharge of the capacitor 20 to the magnetron I0.

The non-linear inductor 22 is so designed that the time that is required for the current change from the point at which it is held by the resistor 25 during the on-time of the tube I9 to the point of saturation is equal to the time required for the capacitor 20 to assume its maximum charge. This can be done by the proper proportioning of th'e volume and cross-sectional area of the core and number of turns of the winding, and these dimensions may be so chosen that the magnetization of the coil changes from substantially saturation in one direction to saturation in the direction during the energy transfer interval.

It will be observed that the `values of the capacitor 25 and resistor 26 must each be a compromise. llhus the capacitor 25 must be small enough that the charge it accumulates during the time the tube I9 is cut oli will not drive the non-linear inductor 22 very far into the saturated region when dissipated during the on-time of the tube I9. Also the capacitor 25 must be suiiiciently large compared to the capacitor 20 so that appreciable energy will not be lost during the pulse discharge. Similarly resistor 26 must be sufiiciently large to limit the on-time current through the inductor 22 to a value below saturation and yet have a suiliciently low resistance to permit the current to build up to a. point approaching that saturation value. It is possible to meet these requirements with relatively uncritlcal values for both elements.

In one embodiment of the invention which has given highly satisfactory operation in actual prac tice the following circuit constants were used:

Source or battery 17:1200 volts Inductor 18:220 millihenries Inductor 22 in unsaturated region=1-1.3 henries Inductor 22 in saturatedregion=negligible Capacitor 20:22() micromicroiarads Capacitor 25:0.0025 microfarad Resistor 26=18,000 ohms Charging period (t1-t2) :approximately l'ii microseconds Pulse rate=approximately 1650 per second.

What is claimed is: l

1. In a recurrent pulse generator, a. source of direct current, a first inductor, a storage capaci. tor, switching means for intermittently establishm ing a flow of current from said source through said inductor, circuit connections for charging said storage capacitor by the surge developed upon the interruption oi said flow of current, a second Inductor having a readily saturable core, circuit connections including a bypass capacitor for discharging said storage capacitor to the load through said second inductor, the capacity of said second capacitor being sufilciently low that said second lnductor does not become saturated during the period of said flow oi current, and a resistor connecting said source to said second inductor to supply current thereto through said switching means.

2. A recurrent pulse generator comprising a load, a source of direct current, a linear indue tor, a storage capacitor, switching means for .inrn termittently establishing a flow of current from said source through said linear inductor, circuit connections for charglng'said storage capacitor by the surge developed upon the interruption of said flow of current. a non-linear inductor, a resistor connecting said non-1inear inductor to said source through said switching means, said resistor being of such a value as to limit the current thrOugh'said non-linear inductor to approximately the saturation value, and circuit connections including a lay-pass capacitor for discharging said storage capacitor to the load.

3. A recurrent pulse generator in accordance with claim 2 in which said blocking capacitor is of such value` that the charge accumulated therei1 by during the inactive portion of the cycle cf operation is readily dissipated through switching means during the active period.

4. A recurrent pulse generator comprising' load, a source of direct current, a linear induct a nondinear inductor, switching means for termi'ttently establishing a flow current irs said source through said incluctors in parallel, a storage capacitor, circuit connections 'for charsw ing said storage capacitor by the surge developed upon the ii'lterruption of said fioul' oi current, and other circuit connections for discharging said storage capacitor to the 'load through said nouw linear inductor.

A recurrent pulse generator according to claim l including a resistor connected in series with said non linear inductnr ior the flow of current from said source and having such a value as to limit the current ilow through said nonnlinear inductor from said source to a value approx .imating the saturation value.

I nonelin 61A recurrent pulse generator according to claim i including in said other circuit connec tions a .bypass capacitor 'in series with said nom linear inductor.

7. A recurrent puise generator in accordance with claim. i including is series with said nomiinear inductor i :he iio'w of current from said source having such. a value as to limit the 'current fio rough said non-'linear inductor to approximately the sii-tw ration value and further inciuding e. hy-pass ca pacitor connected in. series with said nonmiineer inductor for the discharge current from said ster age capacitor.

8. A .remirrcntv pulse generator comprising a source of direct current, alinear inductor, a steru age capacitor, a load circuit having an acyrnrnetrical conduction characteristic, a charging cincuit having an asymmetrical conductor charac teristle and so connected in shunt to said load circuit as to provide a path for current in the direction opposite to that provided by said load circuit, switching means for periodically causing a current from said source through said linear inductor to store energy therein -and for inter" rupting said flow of current whereby said eru'erry is transferred to saisi storage capacitor through said charging. circuit, a non-linear inductie,Y a direct current circuit for supplying current from said source to said nondinear lnductor through itching means, and surge circuit for g: said storage capacitor through said .i inductor to said load circuit.

c., ii. recurrent puise generator according' to claim including in said direct current circuit a resistor of such 'a value as to iirnit the direct current from said source through said nou-lin ear :indue-tor 'to approximately the saturation value.

iii. il recurrent pulse generator according to claim 8 including in said surge circuit a ca pacitor of 4such value that its charge is readily dissipated during the period of current flow from said. source to linear inductor.

ll. A recurrent pulse generator according to claim c including sait?. direct c .t circuit resistor for limit/iu the current rraid source through said non-linear induetor and further in clucling in said surge c cuit a capacitor of such vaine that its charge cartes. during the period ci current iiow from said source to said linear inductol. l

reci went 'puise generator coiilnrisinsr a source of direct current, a linear inductor, a

storage capacitor, a nonlinear incliictor, a re sister, nonmlinear ins'iuotor and said resistor losing connected in series with each other in nur allel to said linear inductor, switching means for intermittently establishing a flow of direct our rent from said source through said inductors in. parallel, the value ci said resistor such to limit the current through no linear im ductor to approximately the saturation. point in one sense, circuit connections for charging said storage capacitor by the surge deveioiceu upon the interruption of said ilow of current, and circuit connections including a bjr-pass capacitor for die charging said storage vcapacitor to the load through said non-linear inductcr.

A. recurrent pulse concierto, claim in which :fi itirigl the current ilovv said nonuiinear inductor to approximately' the saturation value,

ifi. A. recurrent pulse wir according tu dum 12 m which um by-nu ma nu 8 nid non-linear Inductor to approximately the ntuntlon point end in which nld by-pnu ce. pldtorhasmchsvduethxttcchsrgelsdlsslpsted during the period ot current now from 5 nldmrtoidlnductcr.

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